1. Field of the Invention
The invention relates to a discharge lamp of the short arc type which is used as the light source of a projection device.
2. Description of the Related Art
Conventionally, the discharge lamp of the short arc type shown in FIG. 4 is known as the light source of a projection device such as a projector or the like. In this discharge lamp, bordering the two ends of the arc tube 1 of silica glass, are two hermetically sealed tubes 2 of silica glass. In the arc tube 1, there is a pair of tungsten electrodes 3 and lead pins 4 of tungsten, each supporting an electrode 3 and sealed by a graded glass 5. “Graded glass” is defined as glass with a changing coefficient of thermal expansion. If necessary, there can be a base 6 on the end of the hermetically sealed tube 2.
In such a discharge lamp of the short arc type, the internal pressure of the arc tube 1 is very high during operation in order to increase the radiance. Therefore, it is necessary to design the lamp such that the hermetically sealed tubes 2 are not damaged even at a high internal pressure. Furthermore, since a high current flows in the lamp, it is necessary for the respective lead pin 4 which supports the electrode 3 to project directly to the outside from the hermetically sealed tube 2 which adjoins the arc tube 1. For hermetic sealing of the hermetically sealed tube 2 on the lead pin 4, there is a hermetically sealed arrangement in which graded glass 5 is used.
FIG. 5 is a schematic cross section which shows the relation between the discharge lamp of the short arc type, a reflector and a light exit window when such a discharge lamp of the short arc type is installed in a projection device. In the projection device, there is a reflector 71 with a rotationally elliptical reflection surface and a reflector 72 with a rotationally spherical reflection surface arranged such that they surround the arc tube 1. The lamp is arranged such that the arc which forms between the electrodes 3 is located at the focal point of the reflector 71. The light which forms in the arc is focused by means of the reflector 71 and is reflected in the direction to the light exit window 8. The light deviating from the reflector 71 is captured by the reflector 72 and reflected back in the direction to the reflector 71 so that the light is efficiently used. The focused light which has been reflected by the reflectors 71, 72 passes through the light exit window 8 and irradiates a film, liquid crystal and the like.
However, as is shown in FIG. 5, the arc which forms between the electrodes 3 has a certain broadening. Part of the light which has formed in the arc and which is at a location which deviates from the focal point of the reflector 71 is reflected by means of the reflector 71 and is returned to the electrode 3. The light which has been reflected by the electrode 3 is radiated again onto the reflector 71. The light which has been reflected by means of the reflector 71 is emitted onto the hermetically sealed tube 2.
The light which has been emitted onto the hermetically sealed tube 2 passes through the hermetically sealed tube 2 and is emitted onto the lead pin 4 and the graded glass 5 since the hermetically sealed tube 2 is made of transparent glass. In particular, when the light is emitted onto the lead pin 4, the lead pin 4 is heated by the light, which causes its temperature to rise because the lead pin 4 is a component which does not transmit light.
Some of the light which has been formed in the arc at the location which deviates from the focal point of the reflector 71 is reflected by means of the reflector 71, is emitted directly onto the hermetically sealed tube 2 and heats the lead pin 4 (the light beam is, however, not shown in the drawings). Furthermore, part of the light which has been reflected by the surface of the light exit window 8 is emitted directly onto the hermetically sealed tube 2 and heats the lead pin 4 (not shown in the drawings either).
As shown in FIG. 6, the light which has penetrated into the arc tube 1 with a certain angular range is relayed within the arc tube 1 and within the hermetically sealed tube 2. In this way the relayed light L4 reaches the lead pin 4, or the light L3 which has emerged from the inside of the hermetically sealed tube 2 in the direction to the lead pin 4 is emitted onto the lead pin 4 and heats the latter. Furthermore, there is light L2 which emerges from the inside of the hermetically sealed tube 2 in the direction out of the lamp.
If the temperature of the lead pin 4 increases in this way by the irradiation of the lead pin 4 with light, the heat of the lead pin 4 is conducted to the graded glass 5 which is sealed on the lead pin 4. In this way, the disadvantage occurs that the temperature of the graded glass 5 rises, which results in stress in the graded glass 5 such that eventually cracks form and that the hermetically sealed arrangement is destroyed.
Furthermore, there is the following disadvantage that the end of the lead pin 4 projects to the outside from the end of the hermetically sealed tube 2. Since the projecting part is exposed to outside air, oxidation of the lead pin 4 continues when the lead pin 4 reaches a high temperature. As a result, expansion of the lead pin 4 occurs, and cracks form in the graded glass 5 in contact with the lead pin 4. The hermetically sealed arrangement is therefore destroyed.